the POWER (voltage and current) used by the volt meter is less than the self discharge rate of the battery and in general negligable. I use the mini-meters on eBay for about $1.50 US.

I have a big issue with your on/off switch. Using your link for the charge controller (regulator) the instructions say connect battery first. THIS IS VERY IMPORTANT.I have destroyed several 'regulators' by inadvertntly removing the battery first when disconnecting. (or, inversly connecting the solar panel first). If you insist on this switch then you need a double pole switch to also disconnect the solar panel first. Or, more practically, leave the panel connected all the time and put the switch before all the loads. I also disagree with the person below that says the regualor is not needed. This item will prevent you from over discharging (and killing) the battery, and prevent you from over charging the battery, and will include the diode mentioned.